Problem: In physics class, the girl to boy ratio is $2$ to $3$. If there are a total of $20$ students, how many boys are there?
Answer: A ratio of $2$ girls to $3$ boys means that a set of $5$ students will have $2$ girls and $3$ boys. A class of $20$ students has $4$ sets of $5$ students. Because we know that there are $3$ boys in each set of $5$ students, the class must have $4$ groups of $3$ boys each. There is a total of $12$ boys in physics class.